1. Field of the Invention
This invention relates to displacement measuring instruments, and particularly to improvements in a displacement measuring instrument comprising a case having a closed construction, a fixed scale solidly secured to the case, a spindle abutted at the forward end thereof against an object under measurement and reciprocated due to a displacement of the object under measurement and a movable scale operationally associated with the spindle, and capable of measuring the displacement of the object under measurement from a variation in physical value between the movable scale and the fixed scale due to the displacement of the object under measurement.
2. Description of the Prior Art
In general, in a displacement measuring instrument for measuring a length or the like of a product, in the case of a value of movement is measured between relatively movable bodies, for example a value of movement of a measuring element with respect to its main body, or a value of movement of a slider with respect to a column, there has been known to be used a photoelectrical displacement measuring instrument in which a main scale is fixed to one body and a detector including an index scale is fixed to the other body, and a value of relative displacement between the main scale and the detector is photoelectrically read.
In the photoelectrical displacement measuring instrument of the type described, there has been normally used a photoelectrical detector as shown in FIG. 1. In the drawing, designated at reference numeral 10 is a light source, 12 and 14 a main scale and an index scale movable relative to each other, by reciprocating the main scale 12 in the direction shown by arrow A in association with the displacement of the object under measurement for example, in both of which light transmitting portions and non-light transmitting portions are formed in fringe patterns disposed at regular intervals on flat glass plates, respectively, and 16 a light receiving element for receiving a light, which has been emitted from the light source 10 and transmitted through the main scale 12 and the index scale 14. For example, the light source 10, the index scale 14 and the light receiving element 16 are fixed to the case having a substantially enclosed construction, while the main scale 12 is abutted against at the forward end thereof against the object under measurement and is reciprocated in association with the spindle reciprocated due to the displacement of the object under measurement.
In the displacement measuring instrument having such an photoelectrical detector as described above, when the main scale 12 is displaced due to the displacement of the object under measurement, a value of received light in the light receiving element 16 is valued, so that a value of the relative movement between the main scale 12 and the case can be detected, thereby enabling to obtain a characteristic feature of digitally detecting the displacement of the object under measurement. However, heretofore, the displacement measuring instrument of the type described has presented such a disadvantage that, when the rate of travel of the main scale 12 or the index scale 14 becomes high, miscounts in the movement value of the fringes often take place. For example, when the main scale 12 and the index scale 14 have a fringe width of 10 .mu.m and a minimum reading value of 1 .mu.m, if the rate of travel of the main scale 12 or the index scale 14 exceeds approx. 500-800 mm/sec, then errors in measurement due to miscounts have taken place.
To obviate the abovedescribed disadvantage, heretofore, a damper effect has been added to a release which had been used to operate the spindle or the rate of travel of the main scale and the index scale has been restricted by the utilization of an urging force of a spring. Particularly, in the case of utilizing the spring, even when the rate of travel of the main scale 12 or the index scale 14 is low, the spring works against the movement of the main scale 12 or the index scale 14, whereby such a disadvantage has been presented that a force required for operating the spindle and the like is increased in value even when the restriction of the rate of travel is not required.